feat: implement API-first architecture Phase 1 - Dashboard API

**Architecture Refactor (SOLID Principles):**
- Implement AdminDashboardController (REST API)
- Add dashboard summary endpoint
- Add upcoming filings endpoint
- Add recent inquiries endpoint
- Add monthly statistics endpoint

**Database Layer (Repository Pattern):**
- Extend IInquiryRepository with date range queries
- Implement CountByDateRangeAsync
- Implement CountByStatusAndDateAsync
- Extend InquiryRepository with new methods

**Service Layer (Single Responsibility):**
- Extend AdminDashboardService with API methods
- Add GetRecentInquiriesAsync
- Add GetMonthlyStatsAsync with caching

**Test Coverage:**
- Update FakeInquiryRepository mock with new methods

**SOLID Application:**
✓ Single Responsibility: Each class has one reason to change
✓ Open/Closed: Dashboard API can be extended without modifying existing code
✓ Dependency Inversion: Service depends on Repository abstraction
✓ Interface Segregation: API endpoints are focused and specific

Status: ✓ Compiles successfully (0 errors, 0 warnings)

Next phases:
- Add remaining API controllers (Announcement, Client, FAQ, TaxFiling)
- Refactor Blazor components to use API instead of services
- Implement JWT token refresh mechanism
- Add SignalR for change notifications

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

TaxBaik.Infrastructure/Repositories/InquiryRepository.cs

@@ -74,6 +74,28 @@ public class InquiryRepository(IDbConnectionFactory connectionFactory) : BaseRep
             new { Status = status });
     }
 
+    public async Task<int> CountByDateRangeAsync(DateTime startDate, DateTime endDate, CancellationToken cancellationToken = default)
+    {
+        using var conn = Conn();
+        return await conn.ExecuteScalarAsync<int>(
+            @"SELECT COUNT(*)
+              FROM inquiries
+              WHERE created_at >= @StartDate AND created_at <= @EndDate",
+            new { StartDate = startDate, EndDate = endDate });
+    }
+
+    public async Task<int> CountByStatusAndDateAsync(string status, DateTime startDate, DateTime endDate, CancellationToken cancellationToken = default)
+    {
+        using var conn = Conn();
+        return await conn.ExecuteScalarAsync<int>(
+            @"SELECT COUNT(*)
+              FROM inquiries
+              WHERE status = @Status
+                AND created_at >= @StartDate
+                AND created_at <= @EndDate",
+            new { Status = status, StartDate = startDate, EndDate = endDate });
+    }
+
     public async Task UpdateStatusAsync(int id, string status, CancellationToken cancellationToken = default)
     {
         using var conn = Conn();